Combining parts to a single stl for cheaper printing
Welcome to the wonderful (and often confusing) world of Blender!

So, the first thing you'll need to do is right-click the default cube to select it.*


Hit S6 to scale the cube up by 6. Hit ↩︎ Return to finish. (You'll see Dimensions in the right panel go to 12 in each direction. We're pretending the units are inches.)
Set the cube's maximum draw type to Wire, this will avoid it getting in the way:




Ignore the cube for now.


Now, do you have a number pad? If not, you should probably go into the preferences (⌘, on a mac, ⌃ Ctrl+⌥ Alt+U elsewhere) and turn on Emulate Numpad in the Input tab. Be sure to hit the Save User Settings button, or you'll get a surprise when you restart Blender!
Hit 1 on the numpad, or the regular key if you've got Emulate Numpad turned on. This will align your view so you're looking straight on sideways. Then hit 5 on the numpad to switch to "orthographic mode"; this makes your view flat, like an architectural drawing. (There is no perspective; things that are far away look just as big as things that are up close.)
Turn on snapping with Shift+Tab. If it gives you trouble later, you can turn it off again.
For each case (I'm assuming they're all one piece here, please tell me if not):


Select it by right-clicking.
Hit G to Grab it, and then Z to only move it along the Z axis (up/down).
Move your mouse up/down until the bottom of the case is aligned with the floor, which is that red horizontal line going across your viewport.





Now you need that cube again.


Select it by right-clicking on its wire.
Press GZ and move it up 6 to align it with the floor.
Hit Numpad 7 to switch to top view.
Grab each case, like before except without the Z locking, and make sure it's inside the cube's walls.
If you want them to all be one piece, join them with Ctrl+J.
Delete the default cube with X. Now you're done! You can find STL export under the File menu.




Please note that I used A+B to mean "press B while holding A", and AB to mean "press A, then B".



*Yes, it's weird, but you get used to it. If you absolutely hate it, you can change it in the preferences. Just make sure to hit "save user settings" afterward.